1. Field of the Invention
This invention generally relates to a method and an apparatus for testing the insulation condition of a winding portion in an electric device, such as a compact motor or a transformer.
2. Prior Art
In general, the life of a compact motor or a transformer may be fatally shortened due to electric breakdown in its coil when they are in operation. This kind of electric breakdown will be, for example, caused due to the damage of insulating sheath covering a magnet wire of the coil, since such a damage possibly causes a local breakdown (so-called "layer short-circuit") between the magnet wires having different electric potentials. Furthermore, if an insulating sheet interposed between the coil and its iron core is damaged, similar breakdown (so-called "grounded short-circuit") will be caused and an extraordinary current may flow.
An impulse-voltage testing method is a representative method conventionally well-known and used for testing this kind of insulation condition.
Hereinafter, one conventional impulse-voltage testing method will be explained. FIG. 3 is a circuit diagram showing the conventional impulse-voltage testing apparatus. As shown in FIG. 3, a testing apparatus comprises an impulse-voltage generating unit 1 for generating an impulse voltage used for the insulation test, a changeover switch 2 for switching the polarity of the impulse voltage, a measured body 3 consisting of a test-piece coil 30 and a standard (or reference) coil 32 connected in series each other, and an oscilloscope 4 for monitoring the waveform of a connecting point between test-piece coil 30 and standard coil 32.
Impulse-voltage generating unit 1 comprises a commercial power source unit 11 whose output voltage is adjusted by a voltage adjuster 12 so that a step-up transformer 13 connected to this voltage adjuster 12 can generate a predetermined output voltage in a range of 0 to 3000 V. The boosted output of step-up transformer 13 is applied a half-wave rectification through a diode 14, and is stored in a capacitor 15 as a high tension DC voltage. An impulse trigger generating circuit 18 generates a trigger pulse during a dormant period of the half-wave rectification of diode 14 during which no charge is stored in capacitor 15. An SCR 16 is activated in response to the trigger pulse generated from impulse trigger generating circuit 18, thereby outputting an impulse voltage from the impulse-voltage generating unit 1. A resistance 17 is provided to stabilize the operation of SCR 16.
The impulse voltage, generated from the impulse-voltage generating unit 1 in this manner and having the waveform shown in FIG. 4, is then supplied to the changeover switch 2 to alternately switch the polarity of the impulse voltage, and is finally applied to the measured body 3.
As described above, the measured body 3 comprises a pair of test-piece coil 30 and standard coil 32 connected in series. Oscilloscope 4 is connected to the joint point between test-piece coil 30 and standard coil 32. An iron core 31 of test-piece coil 30 and an iron core 33 of standard coil 32 are grounded respectively.
After passing through changeover switch 2, the impulse voltage first flows across test-piece coil 30 and subsequently flows across standard coil 32, and is then extinguished into earth. In this moment, oscilloscope 4 displays the waveform of a transient voltage appearing between the terminal ends of standard coil 32.
Next, changeover switch 2 is turned over. This time, the impulse voltage first flows across standard coil 32 and subsequently flows across test-piece coil 30 and is then extinguished into earth. In this moment, oscilloscope 4 displays the waveform of a transient voltage appearing between the terminal ends of test-piece coil 30. As a result, the above-described two waveforms of transient voltages of both test-piece coil 30 and standard coil 32 are displayed in an overlapped manner on the screen of oscilloscope 4.
The transient-voltage waveform of test-piece coil 30 is basically identical with the transient-voltage waveform of standard coil 32, unless test-piece coil 30 is defective. In other words, only one waveform is displayed on the screen of oscilloscope 4.
On the other hand, in the event that test-piece coil 30 causes the grounded short-circuit or layer short-circuit, the transient-voltage waveform of test-piece coil 30 is explicitly differentiated from the transient-voltage waveform of standard coil 32. Accordingly, two different waveforms are displayed on the screen of oscilloscope 4. Thus, it becomes possible to detect the presence of the ground short-circuit or layer short-circuit in the test-piece coil 30.
However, according to the above-described conventional testing apparatus, the impulse voltage is set to a predetermined constant value in advance to make a judgement as to whether or not the test-piece coil is durable against this setting voltage. For example, when the impulse voltage is set to 2000 V, a test-piece coil will be acceptable if it is durable against 2000 V, or non-acceptable if it causes the grounded short-circuit or layer short-circuit. In short, this conventional testing apparatus is only effective to judge the acceptability of each test-piece coil, and is useless in detecting a threshold impulse voltage at the level of which the electric breakdown is actually caused.
In view of the stress imparted on the test-piece coil, it is generally desirable to suppress the magnitude of the impulse voltage as small as possible. For example, there will be a need for detecting a repairable damage through this test to return the test-piece coil to the manufacturing line. In such a case, it will be strictly prohibited to apply an unnecessarily large impulse voltage on this test-piece coil.